Iran's nuclear chief said Monday that "terrorists and saboteurs" might have infiltrated the International Atomic Energy Agency in an effort to derail his nation's atomic program. It was Tehran's harshest attack on the integrity of the U.N. organization and its investigation of allegations that Iran is striving to make nuclear arms.

Fereydoun Abbasi also rebuked the United States in comments to the agency's 155-nation general conference, reflecting Iran's determination to continue defying international pressure to curb its nuclear program and cooperate with the agency's inspectors.

Revealing what he said were two sabotage attempts on his country's nuclear program, he challenged the perpetrators to launch new attacks, saying his country is determined to learn how to protect its interests through such assaults.

The defiant speech was bound to give a greater voice to hard-line Israeli leaders who say that both diplomatic efforts and economic penalties have had no effect on Iran, leaving military strikes as the only alternative to stopping it from developing nuclear weapons.

Iran has often warned that any Israeli attack would bring a devastating response, and Abbasi suggested Monday that such strikes would not succeed in slowing down his country's nuclear program. He said without elaboration that experts have "devised certain ways through which nuclear facilities remain intact under missile attacks and raids."

Tehran denies seeking nuclear arms, and Abbasi insisted that his country's nuclear program is aimed only at making reactor fuel and doing medical research.

Tehran has long dismissed suspicions that it may re-engineer its uranium enrichment program from making reactor fuel to produce nuclear warheads and says accusations that it has worked secretly on nuclear arms are based on fabricated U.S. and Israeli intelligence. It also frequently accuses the atomic agency of anti-Iranian bias in its push to ensure that all of Tehran's nuclear activities are peaceful.

But Abbasi's comments Monday were the harshest on the agency itself.

"Terrorists and saboteurs might have intruded the agency and might be making decisions covertly," he said. Citing what he said was an example of sabotage on Aug. 17 at an underground enrichment plant, he said agency inspectors arrived to inspect it shortly after power lines were blown up.

It appeared to be the first mention of a sabotage attack. The plant at Fordo, about 40 miles south of Tehran, is of particular concern to Israel because it is buried deep in a mountainside to protect it from assault. Asked to expand on the sabotage accusation, Abbasi said it was foiled "by using backup batteries and diesel generators" that prevented any disruption to centrifuges used to spin uranium to enriched levels.

Abbasi also said a separate attack on the country's centrifuges - through tiny explosions meant to disable key parts of the machines - was discovered before the timed blasts could go off.